When a discharge lamp such as a metal halide lamp or the like is turned ON in AC mode, it is desirable that, in order to avoid occurrence of a phenomenon in which the lighting frequency resonates with an air flow in a discharge tube (i.e., the so-called acoustic resonance), the discharge lamp should be lit at a frequency of several hundred Hz or less. For example, in an automobile discharge lamp, a recommended value of lighting frequency of the discharge lamp is 250 Hz to 750 Hz. Nevertheless, the existing discharge lamp lighting circuit for the vehicle has a circuit configuration such that an input DC voltage from a battery is raised to a DC voltage necessary for the discharge lamp by a DC/DC converter, and then this DC voltage is AC-converted into the lighting frequency of the discharge lamp by a full-bridge inverter provided at a later stage. Also, a starter circuit generates high-voltage pulses of about 25 kV required to start the discharge lamp. A lighting auxiliary circuit promotes a discharge growth from a glow discharge to an arc discharge by supplying an excessive current immediately after the discharge lamp is started, so that this lighting auxiliary circuit allows instantaneous lighting.
Japanese Patent Document JP-A-11-329777 discloses a discharge lamp lighting circuit for the vehicle equipped with the DC/DC converter and the full-bridge inverter.
However, the conventional discharge lamp lighting circuit has a standardized configuration and contains a large number of large-size electronic components, which prevents a size reduction and a cost reduction of the discharge lamp lighting circuit and also becomes a major cause for the fact that the automobile discharge lamp cannot become sufficiently available.
For example, in the circuit configuration disclosed in JP-A-11-329777, there is a large amount of heat produced by the switching element and the rectifier diode in the DC/DC converter, and the temperature of respective elements tends to rise locally. In addition, according to the same configuration, a driver circuit for driving four switching elements of the full-bridge inverter is needed, which also limits size reduction and cost reduction of the discharge lamp lighting circuit.